The present invention is in the field of photochromic glass and particularly relates to clear, essentially colorless boron-free photochromic glasses exhibiting improved stability and photochromic behavior.
U.S. Pat. No. 3,208,860 is the basic patent relating to silver halide-containing photochromic silicate glass compositions, and describes a large number of alkali borosilicate glasses exhibiting useful photochromic behavior. Initial attempts to reduce the boron content of these glasses were based upon a recognition of the fact that glasses containing large amounts of boron are not readily chemically strengthenable. Thus U.S. Pat. No. 3,656,923 discloses photochromic alkali aluminosilicate glasses wherein the boron content is minimized in order to provide materials which can be chemically strengthened by ion-exchange techniques.
It is difficult to totally eliminate B.sub.2 O.sub.3 from photochromic alkali aluminosilicate glass compositions because boron has an important effect on the development of the photochromic silver halide crystallites, and also appears to inhibit the unwanted reduction of copper and/or silver which can lead to red or pink coloration of the glass. Thus many of the prior art compositions designed for ion exchange strengthening contained at least minor amounts of B.sub.2 O.sub.3, not only to stabilize the glass, but more importantly, to prevent unwanted reduction and to insure the proper development of the photochromic silver halide phase.
Another problem encountered in the manufacture of photochromic aluminosilicates relates to the fact that relatively high melting temperatures must be employed, particularly when reduced quantities of fluxing materials such as fluorine and Li.sub.2 O are utilized. Whereas glass stability may be improved by reducing the quantities of these added fluxes, halide volatilization is substantially increased due to the higher melting temperatures required, and photochromic performance is correspondingly degraded. The manufacture of bromine-containing glasses retaining significant quantities of silver chloride and bromide is particularly difficult when high melting temperatures are employed.